Tris(3,4-dimethylenecyclobuteno)benzene Derivatives with Enhanced Carbon–Carbon Bond-Length Alternation in Annelated Benzene

Computational analyses of the tris(3,4-dimethylenecyclobuteno)benzene molecule indicate that the absence of the equivalent of a HOMO of an etheno group between the internal carbon atoms of the annelating butadieno groups is mainly responsible for the relatively weak cyclohexatriene-like fixation in the central benzene ring. To increase the extent of C–C bond-length alternation in the benzene moiety, two strategies of modification of the annelating groups are proposed to make the presence of the equivalent of a HOMO of an etheno group between the internal carbon atoms of the annelating butadieno groups. The first is to introduce a π-electron-donating heteroatom/group to form a five-membered ring with the annelating butadieno groups (model I). The extent of bond-length alternation could be tuned by the choice of the heteroatom/group in model I. The second is to coordinate with transition-metal groups, such as the irontricarbonyl groups, for the annelating butadieno groups (model II). Computational results predict that molecules generated by using these two strategies could form strong cyclohexatriene-like fixation of the double bonds in the central benzene ring.